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Seasonal changes in the composition of plankton in the lower River Murray reflect disproportionate contributions from the Murray and its major tributary, the Darling River. Flows in the Murray are closely regulated, whereas Darling flows are relatively unimpeded. Typically, Murray flows carry blooms of Cyanobacteria (Anabaena and Anacystis spp.) in summer, and diatoms (Melosira and Cyclotella spp.) in winter and spring. The zooplankton, dominated by endemic microcrustaceans, is derived principally from upstream reservoirs. The most common taxa are eurytopic. although there are seasonal occurrences of cold stenothermal species. The Darling flows typically have a high abiogenic turbidity. low algal biomass, and zooplankton dominated by warm stenothermal pantropical rotifers. Of 133 zooplankton taxa recorded from the lower Murray in 1976-80, more than half were pseudoplanktonic or littoral incursions. Of the dominant potamoplankters, more than half were endemics. Plankton density and diversity were distributed contagiously across- and down-river. Mean zooplankton density at Mannum, S.A., was 119 plankters I-1, and average community species composition was 14.4 rotifers, 6.0 cladocerans and 2.4 copepods. Most common zooplankters were eurytopic and indicative of eutrophic conditions; some were influenced by changes in temperature, turbidity and salinity.

Ecological data are presented on the planktonic Rotifera of the lower tract of the River Murray in South Australia. Thirty-five species are recorded. These are predominantly cosmopolitan limno- plankters; their synecology in the Murray, and that of the zooplankton per se reflects the variability of habitat conditions in a river that is alkaline (pH 7.5-8.7), turbid (10-225 nephelometric turbidity units) and moderately saline (0.2-1.0 g/l).

General physical, chemical and biological information is given for a seasonally inundated billabong (Ox-bow Lake) of the Goulburn River near Alexandra. The microcrustacean fauna (Cladocera, Copepoda, Ostracoda) was rich in species, with 59 species in 44 genera being recorded over 1 year. Statistical analyses showed a significantly clumped distribution, and these apparent habitat preferences are interpreted with reference to the functional morphology of the animals and to weedbed characteristics.

Diagnostic features of Rotifera: Monogononta are summarized, and a dichotomous key to the ploimate rotifers is followed by detailed diagnostic keys to the planktonic and facultatively planktonic representatives of the Epiphanidae (Proalides, Epiphanes, Cyrtonia, Rhinoglena and Microcodides) and Brachionidae (Platyias, Brachionus, Keratella, Kellicottia, Notholca and Anuraeopsis). All species known from Australian inland waters are described and figured, as are some widely distributed taxa not yet recorded from the continent. Distribution data and ecological information are also given.

In this paper, the first of a series providing a systematic revision of all Rotifera now known from Australian inland waters, the common bdelloid rotifers are reviewed. The group is epiphytic or epibenthic in habit, but some species commonly occur in open water of billabongs, lakes and rivers, particularly in association with algal blooms. Frequent occurrence of bdelloids in plankton collections necessitates diagnostic keys to their identification. For the common species only, morphological features of taxonomic significance are described and figured to facilitate further study.

The morphology of limbs I, II and III of D. carinata was examined to ascertain whether they could act as filters for food particles. The dimensions of limb III setules (0.29 m), the intersetular distances (0.23 m), the associated Reynolds number (=10-3) and boundary layers (5-9m) suggest that the filtering function attributed to the limb is unlikely. Photographic data and the distribution of Ankistrodesmus falcatus on the feeding limbs show that limb II is the limb most closely associated with particle capture. The structural complexity of limb II suggests a functional diversity that includes particle capture and the redirection of feeding currents produced by limbs III and IV. Analysis of feeding rates and the size selection of algal particles shows that capture is not a simple mechanical process.

Length-weight relationships, feeding rates, and surface area and morphology of the third and fourth thoracic limbs of Daphnia carinata and Ceriodaphnia quadrangula s.l. were examined. Weight (W) increased between the square and cube of body length (L). W= 7.88 × 10-7 L2.23, W= 1 .7 × 10-6 L2.26 for D. carinata and C. quadrangula s.l., respectively. Both cladocerans had small (<0.3 m) intersetular distances. Surface areas (A) of limbs 3 and 4 increased as a power function of body length: A = 5.35 × 10-2 L2.0 (D. carinata); A = 1 .22 L1.6 (C. quadrangula s.l.). Reynolds number was < 1; boundary layers surrounding the setules overlapped at beat rates of 5 and 16 cycles s-1. Filtering rates (K) for both species increased with increasing body length according to K = 1 .8 × 10-4 L2.0 (D. carinata) and K = 1.3 × 10-5 L2.2 (C. quadrangula s.1.). K also increased as a power function of limb surface area. Filtering rates on Ankistrodesmus falcatus decreased above cell concentrations of c. 104 cells ml-1. Hourly feeding rates (No. of cells per animal) increased with increasing cell concentration up to the incipient limiting concentrations and thereafter remained relatively stable. Weight-specific feeding rates for D. carinata decreased with increasing body size but for C. quadrangula s.l. remained relatively constant. D. carinata positively selected particles in the size range 4.8-7.0 m, whereas C. quadrangula s.l. showed no particle preference within the size range 3-12m. Both cladocerans showed differences in their feeding behaviour when offered a range of phytoplankton species but both fed most efficiently on A. falcatus. Although these cladocerans had limb morphologies that differentiated them from their Northern Hemisphere counterparts, this was not reflected in their feeding behaviour.

A V. ?tertius population in Mt Bold Reservoir was found to be heavily parasitized by the rotifer Ascomorphella volvocicola, which may have led to the collapse of the V. ?tertius population.